1. Field of the Invention
The invention relates to the field of recording or transmitting high-frequency pre-emphasized signals where peak level control is required such as in frequency-modulated (FM) multiplex transmissions.
2. Prior Art
In some bandlimited audio transmission and recording systems (and other systems), high-frequency pre-emphasis is employed for purposes of reducing noise. Such systems often require peak level control, and are characterized by the fact that their overload point is very sudden and must not be exceeded for satisfactory system operation. However, as much as possible of the limited bandwidth must be used if the channel is to be used with maximum efficiency. Among the common systems included within this category are FM multiplex (stereophonic and quadraphonic) broadcast systems, disc recording systems, four-channel disc recording with FM subcarrier (CD-4) systems, optical film recording systems employing variable areas, and frequency-division multiplex telephony systems. Other systems (including non-audio systems) may also fall within this category, particularly when they tolerate some delay distortion and ringing.
Typically the audio signal (or other signal) is applied first to a broadband limiter having a finite attack time. The output of the limiter is coupled to a pre-emphasis filter to produce the desired high-frequency pre-emphasis. In the case of FM multiplex stereo broadcast systems in the United States, this pre-emphasis is set by federal regulations at 75 microsecond, single time constant pre-emphasis. To prevent channel overloading with signals containing significant high frequency components, the filter output in some systems is applied to a feedback-controlled lowpass filter to partially cancel the effects of the pre-emphasis.
The pre-emphasized signal requires instantaneous amplitude limiting since the broadband limiter (and feedback-controlled lowpass filter for systems employing such devices) have finite attack or rise times. Without such instantaneous limiting or clipping amplitude overshooting or channel overloading occurs. However, instantaneous limiting or clipping introduces troublesome high frequency components. Specifically, these high frequency components in a United States FM stereo broadcast system could cause interference with the 19kHz pilot tone, and interference between the main channel and subchannel. To avoid this problem and to eliminate high-frequency components in the input signal (which could also cause interference) an anti-aliasing filter is used before the transmission channel to reduce the high frequency components. These filters are generally sharp-cutoff, lowpass filters such as "elliptic function" filters. Again referring to United States FM stereo broadcast practices, these high performance filters are relatively flat to 15kHz and are down 50-60DB at 19kHz.
The use of the anti-aliasing filter introduces a new problem. The "ringing" and phase shifting characteristics of such filters causes rises in some amplitude peaks which result in channel overloading. Thus, the effects of clipping, which is employed to eliminate such overloading are partly lost by anti-aliasing filtering. Even for an ideal filter overloading will occur. For example, a Fourier analysis of a square wave shows that the amplitude of the fundamental sine wave exceeds the amplitude of the square wave by 2.1dB. Thus, when clipping occurs even for waveforms two octaves below the cutoff frequency of the anti-aliasing filter, substantial overloading or overmodulation occurs since the harmonics are stripped from the clipped waveform. For some commercial systems as much as 130%, or more, overmodulation for some conditions is experienced. To compensate for this problem, the dynamic range of the modulated signal is lowered to less than the maximum available range. This results in loss of volume range and, of course, loss in the quality of the transmitted audio signal in the case of FM broadcasting.
As will be seen, the present invention eliminates much of the prior art overloading due to the anti-aliasing low pass filter. Also some of the distortions caused by many existing overload protection devices, including perceivable high-frequency losses, are lessened. Moreover, the presently disclosed system reduces original cost, and alignment problems by eliminating some prior art redundant components.
As mentioned in some prior art pre-emphasis schemes, a lowpass, feedback controlled filter is employed following a pre-emphasis filter. In other systems, the signal is first applied to a feedback controlled low pass filter, and then to a pre-emphasis filter. As will be seen, the present invention employs a split signal path that eliminates one filtering element required by the above described prior art techniques, and has other advantages.